Stress relaxation in oriented polypropylene

Abstract

The stress relaxation of polypropylene, oriented by hot drawing, has been studied after applying tensile strains of 5% and 15% at various angles to the draw direction. The stress relaxation experiments were performed at temperatures of 23, 36.5, 55 and 85°C. At the three lower temperatures, the tensile stress, σ, was a linear function of ln( t + c) where t is the time of relaxation and c is a constant, thus following a relationship which has been observed to apply repeatedly to stress relaxation in metals [3]. The results were interpreted in terms of plastic deformation in the form of shear parallel to the molecular axis arising from a single thermally-activated rate process. In order to account for the orientation dependence of the rate of stress relaxation it was necessary to postulate that tensile stress normal to the molecular axis as well as the shear stress parallel to the molecular axis increased the rate of the process, and thus, both a normal stress activation volume and a shear stress activation volume were introduced. These were found to be of similar magnitudes, of the order of the volume of the crystalline unit cell.